Probing the ionic structure of FLiNaK–ZrF(4) salt mixtures by solid-state NMR

In this study, by applying (19)F, (23)Na and (7)Li high-resolution NMR methods, the evolution of the [Zr(x)F(y)](4x−y) local ionic structures in FLiNaK–ZrF(4) salt mixtures were elucidated. K(3)ZrF(7), Na(3)ZrF(7) and Na(7)Zr(6)F(31) crystal phases were identified when the melt salts were being solidified. The distribution of these [Zr(x)F(y)](4x−y) species was dependent on the content of ZrF(4) in FLiNaK eutectic salts. Moreover, K(3)ZrF(7) phase transition from an orthorhombic lattice into a disordered cubic lattice was clarified, thereby causing dynamics of the coordinated F(−) ions to be reduced and the well-ordered crystal lattices to be destroyed. These mentioned results provide a further insight into the Zr–F based ionic structure and the formation of the disordered Zr–F structure in ZrF(4)-based eutectic salts.